Al-Awza'i (may Allah have mercy on him) said: Nafi' (may Allah have mercy on him) narrated the hadith to us, and he narrated it with some addition. Imam al-Bukhari (may Allah have mercy on him) cited his narration as supporting evidence, and al-Walid bin Muslim (may Allah have mercy on him) mentioned that al-Awza'i (may Allah have mercy on him) heard from Nafi' (may Allah have mercy on him).
Yahya bin Ma'in used to claim that al-Awza'i did not hear from Nafi', the freed slave of Ibn Umar, and what follows supports his view.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6467